1. Field of the Invention
The present invention relates to exhaust gas purifying apparatus and method for an internal combustion engine which includes an NOx trap catalyst and, particularly, relates to a technique of determining a degree of a deterioration of NOx trap catalyst.
2. Description of the Related Art
A typical exhaust gas purifying apparatus of a lean burn engine including an NOx trap catalyst is well known. The NOx trap catalyst has qualities different in accordance with the air-fuel ratio of the exhaust gas. Nitrogen oxides (hereinafter, referred to as NOx) in the exhaust gas are trapped when an air-fuel ratio of the exhaust gas is lean. On the other hand, when the air-fuel ratio is returned to a rich, the NOx trap catalyst discharges trapped NOx. A Japanese Patent Application first Publication No. Heisei 11-324654 published on Nov. 26, 1999 exemplifies an evaluation of a performance deterioration of the NOx trap catalyst. In details, air-fuel ratio sensors are disposed at both of upstream side and downstream side of NOx trap catalyst. A time duration from a first time point at which the air-fuel ratio of the exhaust gas is turned to be rich to a second time point at which a reducing agent of the exhaust gas passes the NOx trap catalyst. The determination of NOx trap catalyst is determined according to the detected time duration. The first time point is identified according to an output of the upstream side air-fuel ratio and the second time point is identified according to an output from the downstream side air ratio sensor.